The present invention relates to integrated devices, and, more particularly, to a technique for packaging a semiconductor chip or die.
Miniaturization compels a generalized reduction in the dimensions of integrated devices. These devices are commonly packaged in an encapsulating resin using injection molding during which the two halves of the mold close on the dam bar of the assembling metal frame of the semiconductor chip or die.
According to consolidated assembling techniques, the die is bonded onto a lowered central isle of the die-stamped metal frame so that the upper surface of the die is coplanar to the peripheral portion of the frame where a plurality of leads are patterned by the die-stamping operation. The wires for connecting the metal pads defined on the upper side of the die to the respective leads of the metal frame are then connected by conventional thermosonic techniques. Once the assembling process is completed, the frame carrying the die is placed between the two halves of the mold, and resin is injected into the mold cavity through an inlet channel defined along one side of the mold.
With respect to the inlet channel that directs the resin into the cavity of the mold, the metal frame has a slit. This split is typically between a raised edge of the depressed central isle of the frame onto which the top side of the semiconductor die is bonded, and a dam bar of the patterned metal frame onto which the two halves of the mold close. The resin is injected through an inlet mouth defined in the lower half shell of the mold and flows through the slit. The resin also fills the upper cavity of the mold, thus encapsulating the die and the connecting wires.
Because the gap between the bottom of the depressed central isle of the metal frame and the lower half of the mold is relatively small, the resin injected into the mold preferentially tends to fill the space defined in the upper portion of the mold cavity, which is relatively larger. This may cause an incomplete or partial filling of the restricted space underneath the depressed central pad or isle of the metal frame.
This problem may be overcome by bending upward the edge of the central pad of the metal frame which defines the passage through which the plastic resin flows toward the upper portion of the cavity of the mold. This forms a xe2x80x9cspoilerxe2x80x9d baffle that intercepts and directs part of the fluid resin toward the bottom part of the mold. This promotes the complete filling of the lower restricted space of the mold, i.e., underneath the depressed central pad of the metal frame that supports the semiconductor die. The geometric arrangement of having the upper surface of the semiconductor die coplanar with the peripheral portion of the metal frame is essential to facilitate soldering of the connecting wires.
As dimensions are scaled down, this arrangement becomes insufficient to ensure the complete filling of the increasingly restricted space underneath the depressed central pad of the metal frame that supports the semiconductor die, as graphically shown in FIGS. 1, 2 and 3. Therefore, the upper half shell of the mold may be equipped with a special insert or protrusion in the inlet for directing the resin. The protrusion is shaped to partially restrict an opening of the slit through which the resin flows toward the upper cavity of the mold. This protruding insert reduces the width of the slit through which the resin flows. This known approach is shown in FIGS. 4, 5 and 6.
However, this approach has drawbacks for promoting a complete filling of the mold cavity. The mold no longer closes entirely on an uninterrupted dam bar of the stamped metal frame, but along the side of the mold where the resin enters into the mold. The closure takes place by a direct abutment of the two halves of the mold.
Despite the mechanical precision of the molds, it is almost inevitable resin flashes occur due to an imperfect sealing of the mold. This problem is accentuated by the unavoidable inconstancy of the thickness of the stamped metal frames. The presence of resin flashes due to an imperfect sealing and their breaking off that is likely to occur during the successive finishing steps of the packaging process releases particles of resin. These particles of resin accumulate in delicate automation mechanisms, thus leading to their malfunctioning.
Moreover, the criticality of the design and in the machining of such molds, along with their overall relatively high cost, is easily recognized.
The above noted drawbacks and difficulties are effectively overcome by the present invention, which reduces the width of the resin flow slit that is defined with respect to the inlet zone of the resin. The width is reduced by displacing inward by an adequate distance the edge of the upper half shell of the mold, and along the side of the injection of the resin with respect to the edge of the opposite side of the lower half shell of the mold. This creates a misalignment or mismatch of the two halves of the mold, which essentially defines one side of the packaging resin body of the device.
The inner edge of the upper half of the mold reaches a more inward position than the inner edge of the cooperating opposite side of the lower half of the mold. The result is to efficiently restrict the width of the slit through which the fluid resin flows toward the upper part of the cavity of the mold while maintaining the perimeter sealing of the mold uninterruptingly over the dam bar of the die-stamped assembly metal frame.
The mold according to the present invention has proven itself effective in preventing the occurrence of resin flashes as well as in ensuring a complete filling of all the empty spaces of the cavity of the mold. In particular, the narrow space underneath the depressed central pad of the assembly metal frame is completely filled.